1. Field of the Invention
This invention relates to a process of forming a silicon-based film (a film which comprises silicon), a silicon-based film, a semiconductor device having a semiconductor junction formed of a silicon-based film, and a system for forming a silicon-based film.
2. Related Background Art
High-frequency plasma CVD (chemical vapor deposition) enables large-area film formation and low-temperature film formation with ease and brings about an improvement in process throughput. Accordingly, this is one of means which are reliable for the mass production of silicon-based films.
As semiconductor devices having semiconductor junctions formed of silicon-based films, think about solar cells for example. Compared with the existing energy which utilizes fossil fuel, solar cells making use of silicon-based films have an advantage that the energy source is limitless and an advantage that their electricity generation process is clean.
However, in order to more advance the spread of solar cells, the unit price per electrical quantity (electric energy) of generated electricity must be more lowered. For this end, the rate of film formation in the high-frequency plasma CVD must be improved and also the photoelectric conversion efficiency must be enhanced. How a technique for achieving these be established is one of important technical subjects.
With regard to thin-film transistors used to, e.g., make liquid-crystal display devices drive with matrices, even when their gate width is made finer, the value of electric current that is necessary for circuit drive can sufficiently be ensured as long as a high mobility of carriers has been achieved, so that pixels can be arranged in fine pitches. Hence, devices can be made compact and feasible for highly minute display relatively with ease. Accordingly, it is sought to provide thin-film transistors having characteristics of higher mobility.
In photovoltaic devices having a p-i-n junction, where the i-type semiconductor layer that functions substantially as a light absorption layer is formed as an i-type semiconductor layer containing a crystal phase, there is an advantage that the phenomenon of photodeterioration due to the Staebler-Wronski effect, which comes into question in the case of amorphous components, can be kept from occurring. Hence, it is effective to form any i-type semiconductor layer as the i-type semiconductor layer containing a crystal phase. Also, thin-film transistors formed of the semiconductor layer containing a crystal phase have a mobility which is greater by two figures or more, than thin-film transistors formed of an amorphous-phase semiconductor layer, and hence are expected to be greatly improved in TFT (thin-film transistor) characteristics.
With the foregoing for a background, in recent years, studies are made in variety on techniques concerning how silicon-based films be deposited at a higher rate and techniques concerning the formation of silicon-based films containing a crystal phase.
A high-frequency plasma CVD improved in film formation rate is disclosed in Japanese Patent Publication No. 7-105354. More specifically, it discloses that, where the frequency of high-frequency power is represented by f (MHz) and the distance between a substrate and an electrode by d (cm), in the range where f is 25 to 150 MHz and taking note of the relationship between f and d, it is preferable to form films setting f/d within the range of 30 to 100 MHz and that a method is particularly preferred in which films are formed in the region where d is 1 to 3 cm or the region where pressure is 0.1 to 0.5 mbar.
With regard to processes of forming crystalline silicon-based films, Japanese Patent Application Laid-Open No. 11-330520 discloses that a silicon-based thin-film layer can be formed at a high rate when formed under conditions that silane-based gas and hydrogen gas are used, the internal pressure of a reaction chamber is set at 5 Torr or more and the substrate-electrode distance is 1 cm or less.
As a method of forming a high-quality deposited film, Japanese Patent Application Laid-Open No. 6-97078 also discloses a method in which a voltage making the substrate surface have negative potential is applied to the substrate-side electrode.
In processes for forming silicon-based films on substrates by plasma CVD, it has become known that the film formation rate can be made higher by making higher the high-frequency power to be supplied, making smaller the distance between the substrate and a high-frequency power feed section, and making higher the high-frequency power per plasma space.
However, an increase in the high-frequency power per plasma space results in an increase in electron density in the plasma and concurrently therewith an increase in ion density. Also, positive ions are accelerated by electrostatic attraction in the sheath region in a discharge space. Hence, ion bombardment may take place to strain atomic arrangement in the bulk or cause formation of voids in the film. Thus, making higher the high-frequency power per plasma space can be an inhibitory factor against the formation of high-quality silicon-based films.
In the method disclosed in Japanese Patent Application Laid-Open No. 6-97078, it is also considered difficult to keep the positive ions from ion bombardment, and also it is considered difficult to apply the method to a roll-to-roll method which is a method of forming large-are silicon-based films continuously.
An object of the present invention is to provide a silicon-based film formation process which can form a silicon-based film having less defect density and superior characteristics, at a lower cost and a higher film formation rate than any conventional processes.
Another object of the present invention is to provide a silicon-based film having less defect density.
Still another object of the present invention is to provide a semiconductor device having good electrical characteristics, also having superior adherence to any underlying layer, environmental resistance and so forth, and can enjoy a short tact time at the time of manufacture.
The silicon-based film formation process of the present invention is a process comprising feeding a source gas into a vacuum reactor, and applying a high-frequency power to a high-frequency power feed section installed in the vacuum reactor, to cause plasma to take place to form a silicon-based film by chemical vapor deposition on a substrate placed on an electrode provided at a position opposing the high-frequency power feed section and grounded electrically, and is characterized in that;
the film is formed superimposing a direct-current potential on the high-frequency power to set the potential of the high-frequency power feed section to a potential which is lower by V1 than the ground potential, the V1 satisfying the following expression:
|V2|xe2x89xa6|V1|xe2x89xa650xc3x97|V2|,
where V2 is the potential difference from the ground potential, produced in the electrode in the state the plasma has taken place under the same conditions except that the direct-current potential is not superposed on the high-frequency power and the electrode is brought into a non-grounded state.
The silicon-based film of the present invention is characterized by being formed by the above silicon-based film formation process.
The semiconductor device of the present invention is a semiconductor device having on a substrate a semiconductor junction formed of silicon-based films, and is characterized in that at least one silicon-based film in the semiconductor device is the above silicon-based film.
The silicon-based film formation system of the present invention comprises:
a vacuum reactor;
a gas feed pipe for feeding a source gas into the vacuum reactor;
a high-frequency power feed section installed in the vacuum reactor;
a high-frequency power source for applying a high-frequency power to the high-frequency power feed section;
an electrode provided at a position opposing the high-frequency power feed section; and
a direct-current power source for superposing a direct-current potential on the high-frequency power applied to the high-frequency power feed section; and
is characterized in that the electrode is electrically grounded, and the potential of the high-frequency power feed section is so set as to be a potential which is lower by V1 than the ground potential; the V1 satisfying the following expression:
|V2|xe2x89xa6|V1|xe2x89xa650xc3x97|V2|,
where V2 is the potential difference from the ground potential, produced in the electrode in the state the plasma has taken place under the same conditions except that the direct-current potential is not superposed on the high-frequency power and the electrode is brought into a non-grounded state.
The direct-current potential V1 may preferably be within the range of 2xc3x97|V2|xe2x89xa6|V1|xe2x89xa630xc3x97|V2|. In the high-frequency plasma CVD, the substrate may preferably be so set as to serve also as the electrode provided at a position opposing the high-frequency power feed section and grounded electrically. The high-frequency power may preferably have a frequency of from 20 MHz to 500 MHz. The high-frequency power may more preferably have a frequency of from 30 MHz to 150 MHz. The silicon-based film may preferably be a silicon-based film containing a crystal phase. The silicon-based film containing a crystal phase may preferably contain a region in which the diffraction intensity of the (220)-plane, measured by X-ray or electron-ray diffraction of the crystal phase, is in a proportion of 80% or more with respect to the total diffraction intensity.
The source gas may preferably contain at least one of silicon hydride and silicon fluoride, and hydrogen. The distance between the high-frequency power feed section and the substrate may preferably be from 3 mm to 30 mm. The silicon-based film may preferably be formed under a pressure of from 100 Pa (0.75 Torr) to 5,000 Pa (37.5 Torr). In forming the silicon-based film, the source gas may preferably be fed at a residence time of from 0.01 second to 10 seconds. In forming the silicon-based film, the source gas may more preferably be fed at a residence time of from 0.1 second to 3 seconds. In forming the silicon-based film, the high-frequency power may preferably be applied at a density of from 0.01 W/cm3 to 2 W/cm3. In forming the silicon-based film, the high-frequency power may more preferably be applied at a density of from 0.1 W/cm3 to 1 W/cm3.
The silicon-based film may preferably contain at least one of oxygen atoms, carbon atoms and nitrogen atoms, which may be in a total content of from 1.5xc3x971018 atoms/cm3 to 5.0xc3x971019 atoms/cm3. The silicon-based film may preferably contain fluorine atoms in a content of from 1.0xc3x971019 atoms/cm3 to 2.5xc3x971020 atoms/cm3.
The semiconductor device having a semiconductor junction may preferably be a photovoltaic device having at least one set of a p-i-n semiconductor junction in which a semiconductor layer showing a first conductivity type, an i-type semiconductor layer and a semiconductor layer showing a second conductivity type are superposed in order.